Spaced apart, movable components often undesirably stick together during the release step of the MEMS fabrication process. For example, a finished MEMS device may have a movable member that is spaced from and suspended above an underlying substrate. Prior to release, the movable member may be immovable and spaced from the substrate by an intervening sacrificial oxide layer. To release the movable member, the process may immerse the partially fabricated device in a liquid acid bath, which removes the oxide layer. The liquid surface tension of the acid, however, often causes the movable member to stick to the substrate. This phenomenon, which is known in the art as “stiction,” reduces fabrication yields.
Stiction also can present a problem during use. For example, a MEMS microphone, which often is exposed to the environment, may develop moisture between its movable member (its diaphragm) and its substrate (its backplate).
Those in the art have responded to this problem with a number of different solutions. One such solution coats the exposed surface with a polymer or monomer material. Undesirably, such a material often is not robust and can degrade, consequently causing reliability issues. Other solutions require extensive additions to the fabrication process, thus increasing cost.